This invention relates generally to ink jet printing and, more particularly, to methods and apparatus for generating color balanced ink drops in a drop on demand ink jet printer.
Ink jet printing is a prominent contender in the digitally controlled electronic printing arena in part because of its non-impact and low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop-on-demand ink jet.
Drop-on-demand ink jet printers selectively eject droplets of ink toward a printing media to create an image. Such printers typically include a print head having an array of nozzles, each of which is supplied with ink. Each of the nozzles communicates with a chamber, which can be pressurized in response to an electrical impulse to induce the generation of an ink droplet from the outlet of the nozzle. Many such printers use piezoelectric transducers to create the momentary pressure necessary to generate an ink droplet.
Drop-on-demand printers utilizing thermally-actuated paddles have also been suggested. Each paddle would include two dissimilar metals and a heating element connected thereto. When an electrical pulse is conducted to the heating element, the difference in the coefficient of expansion between the two dissimilar metals causes them to momentarily curl in much the same action as a bimetallic thermometer, only much quicker. A paddle is attached to the dissimilar metals to convert momentary curling action of these metals into a compressive wave that effectively ejects a droplet of ink out of the nozzle outlet.
Printing images in a plurality of colors is highly desirable. This has been effected by means of a plurality of streams of ink droplets emitted from a plurality of nozzles. However, the images produced in this way are in general binary in the sense that the number of colors available for each drop is limited to that of the number of associated ink reservoirs and nozzle sets.
Commonly assigned U.S. Pat. No. 5,606,351, which issued to Gilbert A. Hawkins on Feb. 25, 1997, discloses a system having the ability to control the intensity of color droplets by mixing two or more fluid ink components (dyes, pigments, etc.) drawn into a chamber from refill channels. As such, each ink ejector squirts an ink of a particular color of varying intensity and is not capable of altering the color. That is, only the tone of the color is altered.
Commonly assigned U.S. Pat. No. 6,097,406, which issued to Anthony A. Lubinsky et al. on Aug. 1, 2000, discloses an apparatus for mixing and ejecting mixed colorant drops. A mixing chamber receives the appropriate amounts of primary colors and a drop is ejected. However, a residual amount of dye is left in the chamber and needs to be removed by flushing with a clear cleaning fluid before the next color is prepared. A separate diluent chamber is used to control color density.
Commonly assigned, co-pending U.S. patent application Ser. No. 09/466,977 entitled CONTINUOUS COLOR INK JET PRINT HEAD APPARATUS AND METHOD, filed in the name of John A. Lebens on Dec. 17, 1999, discloses a scheme for color mixing in a continuous ink jet print head. By selectively restricting flow of two or more different color inks to a nozzle, a range of colored inks can be ejected from the nozzle.
U.S. Pat. No. 4,614,953, which issued to James M. Lapeyre on Sep. 30, 1986, discloses a color inkjet printing mechanism in which real time color mixing is achieved in a single channel. The method is said to be applicable to either drop-on-demand or continuous stream inkjet printer heads. According to the Lapeyre patent, the relative sizes of a mixing chamber line and its subsequent drive chamber mixed ink drive interior are such that a continuous flow of in is maintained without significant mixing or blurring of different colors sequentially provided within the ink flow.
U.S. Pat. No. 4,382,262, which issued to Joseph Savit on May 3, 1983, discloses a method for ink jet printing in which a first dye component is printed on a receiver. One of several complementary dye components is selectively provided by dedicated nozzles, thereby producing a selected color.
Commonly assigned U.S. Pat. No. 6,055,004, which issued to Werner Fassler et al. on Apr. 25, 2000, discloses a microfluidic printing array print head. Micropumps are used to deliver various colors into a nozzle area to create a drop of desired color. The colored drop is then transferred to a receiver by contact. A shutter plate is used to control ink flow.
According to a feature of the present invention, a drop-on-demand ink jet printing system includes a print head having at least one mixing chamber having a nozzle opening. A plurality of sources of color liquid ink and a source of colorless liquid ink communicate with the mixing chamber. A flow controller is adapted to selectably meter ink from the sources to the mixing chamber, whereby ink droplets of selectable color are prepared in the mixing chamber for delivery from the nozzle. The flow controller is further adapted to meter colorless ink into the mixing chamber after a droplet is delivered from the nozzle opening to thereby dilute color ink remaining in the mixing chamber sufficiently such that a next desired color can be attained by adding ink of appropriate color to the mixing chamber.
According to one preferred embodiment of the present invention, the mixing chamber is flushed with colorless ink after a droplet is delivered from the nozzle opening and before adding ink of appropriate color to the mixing chamber to attain a next desired color.
According to another preferred embodiment of the present invention, the flushed ink is bleached before being returning to the source of colorless ink.
According to yet another preferred embodiment of the present invention, flushed ink is converted to black by addition of appropriate amounts of color ink before being returning to a source of black ink.
Advantages associated with the present invention include the ability to produce continuous tone images without the associated need to print with smaller drops to avoid image pixels being filled by only one drop. For example, the image pixel of a 300 dpi printer is approximately 84 micron square, requiring a 60 micron diameter drop for a spread factor of two when the drop impacts paper. The nozzle diameter may therefore be close to 60 microns. Such large nozzles are less likely to clog and therefore are more robust. Furthermore, large nozzles are easily cleaned. Large nozzles may also employ more viscous inks putting less demand on ink formulation.
The method of controlling color by adding dye and bleach provides a unique means of obtaining color balance on demand. This method allows single drop per image pixel printing with any color of choice color with many levels of intensity.
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiments presented below.